Downstream bug https://bugzilla.redhat.com/show_bug.cgi?id=1651022 (with screenshot) This seems isolated to 4.13.x panel, not 4.12 or eariler.
Still a problem with xfce4-panel 4.13.5. This makes the vertical panel mode quite unusable.
I'm not sure why ellipsizing was consciously disabled in vertical mode... https://git.xfce.org/xfce/xfce4-panel/tree/plugins/tasklist/tasklist-widget.c#n4340 In any case, I think I can fix that. I'm just wondering if there could be any negative side-effects or if it's simply a Gtk+2 vs. Gtk+3 difference.
Simon Steinbeiss referenced this bugreport in commit 0c80148ae547d9311e9645e276d6d3604b10c535 tasklist: Fix overlapping vertical window titles (Bug #14901) https://git.xfce.org/xfce/xfce4-panel/commit?id=0c80148ae547d9311e9645e276d6d3604b10c535
The issue should be fixed. As I said I'm not sure why ellipsizing was disabled in vertical mode. I can only hope there are no strange side-effects, so please give it some testing!